1. Field of the Invention
The present invention relates to a magnetoresistance effect device for reading an information signal recorded on a magnetic storage medium using a magnetoresistance effect sensor, and a magnetoresistance detection system and magnetic storage system.
2. Description of the Related Art
As prior art there is known a magnetoresistance (MR) sensor or a magnetic reading converter called an MR head. This has the characteristic feature that it can read data from the surface of a magnetic recording medium with high linear density. An MR sensor detects a magnetic field signal by means of resistance change as a function of intensity and direction of magnetic flux sensed by a reading device. Such a prior art MR sensor operates in accordance with the anisotropic magnetoresistance (AMR) effect whereby one component of the resistance of the reading device changes in proportion to the square of the cosine of the angle between the magnetisation direction and the direction of the sensing current that flows in the device.
A more detailed description of the AMR effect is given in the article xe2x80x9cMemory, Storage and Related Applicationsxe2x80x9d IEEE Trans. on Mag. MAG-11, P. 1039 (1975) by D. A. Thompson et al. (Magnetic recorder for storing magnetic data on this magnetic storage medium: Thompson). In a magnetic head using the AMR effect, a vertical bias is often applied in order to suppress Barkhausen noise. Antiferromagnetic materials such as FeMn, NiMn or nickel oxide are often used as materials to apply this vertical bias.
Furthermore, in recent years, a more pronounced magnetoresistance effect has been reported in which resistance change of a laminated magnetic sensor is caused by spin dependent transfer of conduction electrons between magnetic layers through a non-magnetic layer and by spin dependent scattering at the layer boundaries association with this. This magnetoresistance effect is called by various names such as the xe2x80x9cgiant magnetoresistance effectxe2x80x9d or xe2x80x9cspin valve effectxe2x80x9d. Such magnetoresistance sensors may be formed of suitable materials and show a larger resistance change with improved sensitivity compared with sensors in which the AMR effect is employed. In such MR sensors, the in-plane resistance between a pair of ferromagnetic layers separated by a non-magnetic layer changes in proportion to the cosine of the angle between the magnetisation directions of the two layers.
Early Japanese Patent Publication H. 2-61572 discloses a laminated magnetic structure for producing a high MR change generated by antiparallel alignment of magnetisation in magnetic layers. The examples given in this publication of materials that can be used in the laminated structure include ferromagnetic transition metals and alloys. Also, a construction is disclosed in which an antiferromagnetic layer is added to one of at least two ferromagnetic layers separated by an intermediate layer, and it is disclosed that FeMn is suitable as this antiferromagnetic layer.
Early Japanese Patent Publication H. 4-358310 discloses an MR sensor independent of direction of current flow through the sensor having two thin-film layers of ferromagnetic material partitioned by a thin-film layer of non-magnetic metal, wherein the direction of magnetisation of the two ferromagnetic thin-film layers are orthogonal in the case where the applied magnetic field is zero and the resistance between the two non-coupled ferromagnetic layers changes in proportion to the cosine of the angle between the magnetisation directions of the two layers.
Early Japanese Patent Publication H. 6-203340 discloses an MR sensor based on the above effect including two ferromagnetic thin-film layers that are separated by a thin-film layer of non-magnetic metallic material and wherein, when the externally applied magnetic field is zero, the magnetisation of an adjacent antiferromagnetic material layer is maintained perpendicular with respect to the other ferromagnetic material layers.
Early Japanese Patent Publication H. 7-262529 discloses a spin valve magnetoresistance effect device having a construction: first magnetic layer/non-magnetic layer/second magnetic layer/antiferromagnetic layer, using in particular CoZrNb, CoZrMo, FeSiAl, FeSi, or NiFe or material wherein Cr, Mn, Pt, Ni, Cu, Ag, Al, Ti, Fe, Co or Zn is added thereto is employed for the first magnetic layer and the second magnetic layer.
The invention disclosed in Early Japanese Publication H. 7-202292 consists in a plurality of magnetic thin films which are laminated with interposition of non-magnetic layers onto a substrate and wherein antiferromagnetic thin films are provided adjacently to one soft mutually adjacent magnetic thin film with interposition of a non-magnetic thin film.
This is a magnetoresistance effect film wherein, if the biasing magnetic field of this antiferromagnetic thin film is Hr and the coercive force of the other soft magnetic thin film is Hc2, Hc2 less than Hr and wherein the antiferromagnetic material consists of at least one of NiO, CoO, FeO, Fe203, MnO or Cr or a mixture of these.
Also, the invention disclosed in Japanese Patent Application H. 6-214837 and Japanese Patent Application H. 6-269524 consists in a magnetoresistance effect film as described above wherein the antiferromagnetic material is a superlattice selected from at least two of NiO, NixCo1xe2x88x92xO, and CoO.
Also, the invention disclosed in Japanese Patent Application H. 7-11354 consists in a magnetoresistance effect film as described above in which the antiferromagnetic material is a superlattice selected from at least two of NiO, NixCo1xe2x88x92xO (x=0.1xcx9c0.9), or CoO and the atomic number ratio of Ni to Co in this superlattice is at least 1.0.
Also, in published Japanese Patent Application H. 7-136670 it is disclosed that a magnetoresistance effect film as described above is a two-layer film in which the antiferromagnetic material is obtained by lamination of 10 to 40 xc3x85 of CoO onto the antiferromagnetic NiO material. However, in the prior art, a magnetoresistance effect device having the basic structure: /magnetic layer/non-magnetic layer/magnetic layer/antiferromagnetic layer/ or /antiferromagnetic layer/magnetic layer/non-magnetic layer/magnetic layer is subject to the following problems. Specifically, by oxidation of the uppermost layer of the structure by annealing treatment at 200xc2x0 C. or more, the exchange coupling magnetic field Hex or the rate of change of magnetoresistance (MR ratio) is lowered. With a magnetoresistance effect device of this type, an exchange coupling magnetic field is obtained that is applied to the fixed magnetic layer from the antiferromagnetic layer, so heat treatment at a temperature of 200xc2x0 C. or more was often necessary. As a result, oxidation occurred in this step, which adversely affected performance.
Also, even if an antiferromagnetic material is used of a type which does not need heat treatment, at the stage of actually manufacturing the read/write head, a step of curing the resist of the write head section is indispensable. In this step, heat treatment at a temperature of 200xc2x0 C. or more was necessary, so oxidation of the magnetoresistance effect film occurred at the stage of processing to form a magnetic head. Also, when a metal was employed as the protective film, if the film thickness was large, due to the conductivity possessed by the metal, there was the problem that a large sensing current, which did not contribute to a change in magnetoresistance, flowed in the protective film and as a result the sensor output was lowered. Also, if the film thickness was small, the oxidation penetrated through the metallic layer into the magnetoresistance effect section, i.e. it could not serve its function as a protective layer.
An object of the present invention is to provide a magnetoresistance effect device and magnetoresistance effect sensor used by a magnetoresistance detection system and a magnetic storage system which are of excellent reliability by ensuring prevention of oxidation of the magnetoresistance effect device in the heating step of manufacture of the read/write head, a sufficient rate of change of resistance, a sufficiently large exchange coupling magnetic field applied to the fixed magnetic field layer from the antiferromagnetic layer and a sufficiently small coercive force of the free magnetic layer by providing a suitable protective film on the uppermost layer of the magnetoresistance effect device. In order to achieve this object, according to the present invention, in a magnetoresistance effect device having a basic construction wherein there is formed a unit consisting of multi-layer films: /magnetic layer/non-magnetic layer/magnetic layer/antiferromagnetic layer/ or /antiferromagnetic layer/magnetic layer/non-magnetic layer/magnetic layer, in a protective film formed on the magnetoresistance effect device there is employed a film of film thickness at least 2 [nm] and less than 7 [nm] consisting of a metal, oxide material, nitride material, mixture of oxide material and nitride material, metal/oxide double layer film, metal/nitride double layer film, or metal/(mixture of oxide and nitride material) double layer film.
If metal is employed as the protective film, the protective film is electrically conductive, so if the protective film thickness is large, the proportion of sensing current that is branched into the protective film is increased. Since the current flowing through the protective film does not contribute to a change of the magnetoresistance, the rate of change of magnetoresistance of the device is decreased, diminishing the output of the head. If the thickness of the protective film is small, the current flowing through the protective film is not particularly great, so the decrease in output due to branching to the protective film is small. On the other hand, if the film thickness is small, the effectiveness of the protective film in protecting the magnetoresistance device from oxidation is decreased. Consequently, there is an optimum region in the film thickness of the metal protective film.
Oxide or nitride are generally essentially non-conductive, so, even if their film thickness is large, the diminution in the rate of change of magnetoresistance due to branching of the sensing current into the protective film not contributing to change of magnetoresistance, such as happens with a metallic protective film, cannot occur. The film thickness can therefore be set to a large value, so in the high-temperature step in the manufacture of a read/write head, entry of oxygen into the magnetoresistance effect device can be effectively prevented so that finally a large Hex, large MR ratio and small Hc of the free magnetic layer can be obtained.
However, oxide or nitride have poor compatibility at the atomic level at the interface with the element constituted by a /magnetic layer/non-magnetic layer/magnetic layer/antiferromagnetic layer/ or /antiferromagnetic layer/magnetic layer/non-magnetic layer/magnetic layer unit which may result in slight deterioration of performance after the high-temperature process. In this case, better performance can be obtained after the high-temperature process by inserting a metal of good compatibility at the atomic level with both the unit and oxide or nitride between the unit and the oxide or nitride.
By means of the above, a magnetoresistance effect device and a magnetoresistance effect sensor used in a magnetoresistance detection system and magnetic storage system can be obtained whereby excellent characteristics are obtained in regard to output value, output waveform, and bit error rate and which can provide good characteristics also in regard to thermal reliability.